In recent years, demand for an ultra-small microphone module including a MEMS microphone has increased. For example, Japanese Unexamined Patent Publication No. 2011-055087 (Patent Document 1), Japanese Unexamined Patent Publication No. 2015-502693 (Patent Document 2), and Japanese Unexamined Patent Publication No. 2007-295487 (Patent Document 3) disclose a MEMS microphone having a configuration in which a membrane and a back plate are disposed to face each other via an air gap on a silicon substrate. In such a MEMS microphone, a capacitor structure is formed of the membrane and the back plate. When a sound pressure is received and the membrane vibrates, capacitance in the capacitor structure changes. The change in capacitance is converted to an electrical signal and amplified in an ASIC chip.